Injection molding relies on the correct identification for the timing of the transfer from injection velocity control to packing pressure control for the production of a quality article. An early transfer can result in a short shot or a part with non-uniformity due to a portion of the cavity being filled during the packing-holding phase. Alternatively, a later transfer results in a mold flash, excessive residual stresses in the parts, and ejection problems.
Current procedures have identified different criteria based on the measurement of specific variables to determine the correct transfer point for transfer from velocity control to packing pressure control. The variables used have been the injection stroke, injection filling time, cavity pressure, nozzle pressure, nozzle pressure derivative or cavity pressure derivative.
The first two of these measurements are volume based. The accuracy is strongly affected by the leakage of the melt through the check ring valve and are also variable due to changes in melt density or variations in the melt temperature.
If criteria are based on cavity pressure or nozzle pressure, the transfer from injection velocity control to packing pressure control takes place when the measurement of the cavity or nozzle pressure reaches a predetermined value. For a specific molding condition, cavity pressure can indicate the degree of filling. However, the installation of a transducer to measure the pressure in the mold increases the tooling costs and introduces undesirable marks on the surface of the molded parts. The use of nozzle pressure provides a less direct indication of the status of the material in the cavity but does not have the installation problem of the cavity pressure measurement. Regardless, both are variable according to the specific molding conditions such as the material, mold geometry, melt temperature and injection velocity.
As an alternative, the nozzle pressure derivative and cavity pressure derivative have been proposed to overcome some of these problems. A rapid change in pressure is expected when the cavity is approaching being completely filled. Again, criteria based on the cavity pressure derivate require measurement of this pressure and, therefore, runs the risk of leaving marks on the surface of the tooling and increased tooling costs. Therefore, the criteria based on the nozzle pressure derivative may be preferred.
All of these measurements also require the criteria to perform a comparison of the proposed measurement against a predetermined threshold. It is the determination of this threshold which is critical to the success of the operation. The choice of a low threshold leads to an early transfer while a high threshold can lead to a late transfer and, potentially, a mold flash.
Nozzle or cavity derivatives are dependent on molding conditions such as injection velocity, melt temperature, material and mold geometry. Therefore, every time that the molding conditions are changed, the predetermined threshold requires reconsideration. This involves an extensive trial and error process at present.